1. Field of the Invention
The present invention relates to a photo mask, more particularly to a halftone phase shift mask suitable for manufacturing fine devices such as a semiconductor device, a semiconductor integrated circuit device, a superconductor device, a micromachine, and electronic devices, in particular suitable for forming very fine patterns, and a production method of the same.
2. Description of the Prior Art
In the manufacturing of semiconductor integrated circuit (IC) devices, a technique known as lithography is used for transferring very fine circuitry patterns onto a semiconductor wafer. The lithograph technique needs in general a projection exposure apparatus, which loads a photomask and transfers the patterns drawn on the photomask thus loaded onto a semiconductor wafer to form patterns of devices thereon. Since the exposure area of an exposure apparatus with a higher resolution is smaller in size than the area of the wafer 21 to be projected, a wafer surface plane figure will be divided into a plurality of xe2x80x9cshotsxe2x80x9d as is shown in FIG. 2, and will be fed in stepping or scanning by one area after another to expose a plurality of chips 22 thereon. At the exposure, a scribing area 23 for separating chips each other will be provided around each chip.
In recent years, the patterns have been made finer than ever for accommodating the requirement of large-scale, high-integration of devices as well as improved device operation speed. Under such circumstances, the wavelength of the exposing light emitted from the exposure apparatus for use in the pattern formation is being shortened. As an alternative, an exposure technique known as halftone phase shifting is also used. The halftone phase shifter mask is a mask having a translucent film (referred to as a halftone film) formed on a transparent plate for attenuating the exposing light and shifting the phase. The transmissivity of the exposure light through the film is, in general, thought desirable in the range between approximately 1% and 25%. The light transmitted through this film is adjusted so as to have-certain phase difference from the light, that does not pass through the film. The best phase difference for achieving the highest resolution is at 180 degrees and the odd multiples thereof. However, the resolution will be more or less improved when the phase is approximately in 180 degrees xc2x190 degrees. It is known in general that the resolution may be improved by approximately 5% to 20% when using a halftone mask.
In the chip exposure, a certain part of outer frame area belonging to the adjacent shot may be overlapped to the pattern formed. When a photo mask is provided with a shade film having a sufficient shading (shielding) performance such as made of Cr or the like, the light transmitted through the outer frame area may cause probably no problem since its amount is sufficiently small. However, when using a halftone mask, the outer frame area is also a part of the halftone mask and therefore is not a perfect shield. The light transmitted through the outer frame area, which may be attenuated but not shut off, will be superposed on the pattern to be formed. The area having exposed to the superposed light will have such a problem as the thickness loss of the resist to be exposed or the decreased resolution. In accordance with the prior art technique, a shade film made of Cr was formed on the outer frame area to solve this problem. The shading film made of Cr is referred to as xe2x80x98Cr shade bandxe2x80x99
An exemplary structure of a mask having the Cr shade band is shown in FIG. 3. In the figure, FIG. 3(a) is a plan view of a mask, and FIG. 3(b) is a cross-sectional of the mask taken along with the line A-Axe2x80x2. A desired pattern 31 to be delineated is placed on a chip field 32 made from a halftone film (the chip field is a pattern forming area, corresponding to a circuit pattern area in manufacturing semiconductor devices). On the entire outer field surrounding the chip field 32, a Cr shade film 33 is deposited. In this figure, the reference numeral 36 designates a transparent plate.
The pattern on this chip field 32 will be delineated on the wafer by exposing it to form a chip 22 shown in FIG. 2. If the Cr shade film 33 is not deposited, the exposure will be superposed on the area 34 in the vicinity of the outer periphery of the chip field. This may happen because of foggy effect by exposure in the peripheral region. Although, in general, the exposure apparatus has a masking blade for adjusting the area size of exposure field, the precision of position alignment is as low as on the order of about 50 micrometers, and in addition, the inherent lack of sufficient sharpness of shielding characteristics of the masking blade cause the foggy effect. The multiple exposures on the outer periphery of the chip field may result in a poor resolution. On the other hand, when a Cr shade film 33 is applied to the entire surface area of the peripheral region as shown in FIG. 3, such problems may not happen as the exposure light will be shielded with a sufficient sharpness at the border.
As an alternative to the use of Cr shade band, a halftone shade band method has been proposed, in which a dense grating pattern or a checker pattern may be cut on the halftone film, the pattern being finer than the resolution of the exposure apparatus used in order to well decrease the transmissivity of the exposure light passing through the patterned area by making use of the diffraction. The method above is disclosed in the Japanese Unexamined Patent Publication No. Hei 06-175347.
References on the halftone phase shift include the Japanese Unexamined Patent Publication No. Hei 05-181257, which disclosure is incorporated herein as a reference.
Now referring to FIG. 4, the procedural steps of producing a halftone phase shift mask with a Cr shade band is shown according to a prototype experiment conducted by the inventors of the present invention. As shown in FIG. 4(a), on a quartz glass plate 40 a halftone film 41, a Cr film 42, and a resist film 43 in the order were sequentially deposited prior to exposing a desired pattern (44). It is to be noted here that the Cr film 42 was formed by the sputter technology. Then the plate was developed to form a resist pattern 43xe2x80x2 shown in FIG. 4(b). Then, the exposed Cr film 42 and halftone film 41 were etched successively to develop Cr pattern 42xe2x80x2 and halftone film pattern 41xe2x80x2 as shown in FIG. 4(c). Next, after removing the resist 43xe2x80x2 as in FIG. 4(d), resist 45 was again applied thereto as shown in FIG. 4(e) and the chip field was exposed (46). Then the development lead to the resist pattern 45xe2x80x2 formed as shown in FIG. 4(f), while Cr film remaining on the surface unmasked by the resist was removed by etching to form the Cr pattern 42xe2x80x3 as shown in FIG. 4(g). Finally the resist 45xe2x80x2 was removed to obtain the halftone phase shifting mask as shown in FIG. 4(h) (which corresponds to FIG. 3(b)).
The inventors of the present invention have found that, after the experimental production cited above, there exist problems as follows. As the halftone phase shift mask using the Cr shade band has a structure in which the Cr film is deposited thereon in addition to the halftone film, (1) there are required a number of steps in manufacturing the mask, causing the high cost; (2) in addition to the considerable number of steps, the process contains such steps as sputtering and etching of the Cr film, which may often cause some defects by a particle, resulting in a lower yield of mask manufacturing; (3) in the process steps from FIGS. 4(f) to (g), the decreased phase controllability of the halftone film may be resulted by partly inhomogeneous etch during the removal of the Cr film that covers the fine-patterned halftone phase shift mask, as well as the accuracy of sizing the patterns may be decreased; (4) a material for the halftone mask should be chosen to have large selection ratio of etching compared to the Cr while at the same time the Cr film and the halftone film must be etched at a higher accuracy after compilation, therefore, the improvement of accuracy is limited by the narrow range of selectable materials. The limited range of selectable materials may cause a severe deficiency in a mask for ArF excimer laser (at the wavelength of 193 nanometers) and F2 excimer laser (at the wavelength of 157 nanometers), since both lasers emit potentially higher energy in the exposure, encountering with a problem of exposing dose tolerance of the mask.
When a halftone phase shift mask making use of a halftone shade band in lieu of the problematic Cr shade band is used, the number of steps in the manufacturing process can be decreased. However, the time required for delineating a mask will be significantly augmented due to a large number of fine patterns to be formed. Furthermore, the defect inspection of masks will also need a considerable time.
The present invention has an object to overcome the above problems and to provide an improved fine pattern mask.
A more specific object of the present invention is to provide an improved fine and precise pattern mask to allow highly accurate delineation when a plurality of multi exposures are performed with respect to an outer frame area of the very fine pattern by using a halftone phase shift-mask, as is done in the step-and-repeat process.
It is another object of the present invention to provide a method of manufacturing masks to allow the improved fine pattern masks to be produced at lesser costs with positive reproductivity.
Some typical aspects in accordance with the invention disclosed herein will be overviewed in brief below.
The phase shift mask in accordance with the present invention comprises, a phase shift film pattern constituted of a half tone film provided on a transparent plate for attenuating the incident exposure light and shifting the phase of the light, and a resist film layer provided on the outside of and surrounding a chip field on which the phase shift film pattern is formed, and partially covering the top surface of the transparent plate.
The resist film provided outwardly surrounding the chip field may function as a good shade band so as to efficiently prevent thickness loss of the resist film and degraded resolution in the area subject to receive multi exposures at the time of mask pattern delineation, thereby avoiding the problem of deposition and processing of Cr film.
When the phase shift mask incorporates a negative-type resist film in which the exposed area is left over, the manufacturing process of the mask will be further simplified.
In order not to deposit the resist film on the area in contact with for example the stage of exposure apparatus or the transporter system, the resist film will be formed only on some part of adjacent periphery approaching to the circumferences to the chip field, except for the fringed border surface of the transparent plate constituting the mask to solve the problem encountered at the time of loading into the exposure apparatus.
To achieve this, a pellicle for protecting from particle defects may be adhered on the mask plate by means of a pellicle frame, such that no resist film will be formed at the end section of the pellicle frame to be secured with the mask plate as well as at the outward circumferences of the pellicle.
It should be noted here that the present invention is drastically different from any other mask production methods. For example, the Japanese Unexamined Patent Publication No. Hei 05-289307 discloses a method for forming the mask pattern itself with the resist film, for the purpose of simplifying and improving the precision of the ordinary production process of photomasks. However the latter method uses a binary mask comprised of a transparent section for transmitting the exposure light and a sufficient shade, which mask has inherently no problem concerning the overlapping of exposure light in the outer frame.
The Japanese Unexamined Patent Publication No. Hei 09-211837 discloses an application of resist shade member to the halftone phase mask. This application is for preventing sub-peak delineation within the chip field and forming a halftone mask of rim type having halftone area only in the vicinity of the pattern edges. Here, the present invention has different objects and effects from the method cited just above, and placement of the formed resist is different therefrom. In addition, the method according to the disclosure in the Japanese Unexamined Patent Publication No. Hei 09-211837 has a disadvantage in that there is a limitation on the configuration of the rim width. The present invention is also different therefrom in that the halftone is applied not only to the rim but also to other area. The above-mentioned patent application uses, of necessity, a positive resist required from the limitation in the production process to form the shader member on the halftone film, while, on the other hand, the present invention capable of making use of a negative resist advantageously.
The Japanese Unexamined Patent Publication No. Hei 09-211837 may encounter a limitation of the film thickness of the resist on a mask because the resist is deposited on the circuit pattern area containing very fine patterns. With a thicker film, the resist may act as a wall to affect the delineation characteristics. More specifically, the fidelity to the mask size in the accuracy of size of the delineated patterns will be degraded along with the decrease of threshold resolution. This phenomenon may be prominently seen when off-axis illumination or a lens, having a larger NA (numerical aperture) is utlized.
The present invention, as have been described above, has no limitation in the thickness of the resist film since in accordance with the present invention the resist patterns are formed on the area having no pattern, in the vicinity of shots, i.e., outside of the chip field, instead of forming on the very fine and precise pattern area.
In addition, the present invention may be further characterized in that the absorbance (extinction coefficient) of the resist shade band may be suppressed to a reasonable level by making the resist film thickness larger than the thickness of the halftone film. No degradation of resolution will be occurred because, if a trace of resist is left over on the area of very fine patterns such as circuit patterns of semiconductor IC devices and the like, the residue will not alter the luminous energy of exposure light. In case in which the residue of resist may be left over at a level affecting the resolution, a fluorescence inspection that is simpler and faster than the usual inspection may readily employed to detect any defects caused by the residue. The fluorescence inspection will be described in greater details later.
The above and further objects, additional advantages and novel features of the present invention will be more fully described in following detailed description when the same is read in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are for the purpose of illustration only and not intended as a definition of the present invention.